Geology Reference
In-Depth Information
Ice Factor: The Next Fifty Thousand Years
For the foreseeable future, the biggest determining factor in Earth's continental contours is
ice.Onshorttimescalesofafewhundredorthousandyears,thedepthoftheoceansismost
closelytiedtothetotalvolumeofEarth'sfrozenwater,includingtheicecaps,glaciers,and
continental ice sheets. It's a simple equation: the greater the volume of water tied up in ice
on land, the lower the level of the sea.
The past is key to predicting the future, but how can we possibly know the depth of his-
toric oceans? Satellite observations of ocean levels, though incredibly accurate, are restric-
ted toabout the past two decades. Tide gauge measurements, thoughless accurate andsub-
jecttolocalidiosyncrasies,gobackperhapsacenturyandahalf.Coastalgeologistscanre-
sort to mapping ancient shoreline markers—raised beach terraces, for example, which can
be found in near-shore accumulations of sediments dating back tens of thousands of years,
though such elevated features can reliably reveal only periods of higher water levels. The
positionsoffossilcorals,whichmusthavegrownintheocean'sshallowsunshinezone,can
push the record even further back, but such rock formations commonly experience epis-
odes of uplift, subsidence, or tilting that confuse the record.
Many scientists now focus on a less obvious indicator of sea level—the variable ratio of
theisotopesofoxygenintinyseashells.Suchratiostellusmuch,muchmorethanacosmic
body'sdistancefromtheSun,asdiscussedin
chapter2
.Becauseoftheirtemperature-sens-
itive nature, oxygen isotopes are also the key to deciphering the historic volume of Earth's
ice and thus ancient sea level changes.
Even so, the connection between ice volume and oxygen isotopes is tricky. By far the
most abundant oxygen isotope, accounting for about 99.8 percent of what we breathe, is
the lighter oxygen-16 (with eight protons and eight neutrons). About one in five hundred
oxygen atoms is the heavier oxygen-18 (with eight protons and ten neutrons). That means
about one in every five hundred water molecules in the ocean is heavier than the average.
As the Sun heats equatorial ocean water, water with the lighter oxygen-16 isotope evapor-
ates a bit faster than that with oxygen-18, ensuring that water in low-latitude clouds is a bit
lighter on average than the oceans from which it came. As the clouds rise to cooler zones,
water with the heavier oxygen-18 isotope condenses into raindrops a bit faster than that
with oxygen-16, ensuring that the cloud's oxygen becomes even lighter than it was before.
By the time the clouds move toward the poles, as clouds inevitably do, the oxygen in their
water molecules has become much lighter than in those of ocean water. When these polar
clouds release their precipitation onto ice caps and glaciers, more of the lighter isotope is
locked in the ice, leaving the oceans heavier.
During times of maximum global cooling, when more than 5 percent of Earth's water
can be frozen solid, the oceans become significantly enriched in oxygen-18. During times
of global warming and glacial retreat, oxygen-18 levels in the oceans decline. So it is that
